This invention relates generally to a power supply and more particularly to the detection and monitoring of the magnitude and shape of the input voltage waveform for a switching power converter.
Switching power converters are designed to receive unregulated alternating current (AC) input power and provide regulated outputs to loads such as electronic devices. Typically, a low frequency AC power source (e.g., 90-270 Volts AC (VAC) at 50-65 Hertz (Hz)) is rectified to provide an unregulated direct current (DC) power source, which is input to a power stage of a switching power converter. In turn, the power stage provides a regulated DC power source to the load.
FIG. 1 illustrates a conventional switching power converter 100 topology for delivering electrical power from an AC power source 104 to a load 107. The AC input source 104 is coupled to a rectifier 101 which converts the AC input into an unregulated DC output 102. The power conversion stage 105 is coupled to receive the unregulated DC output 102 from the rectifier 101 and, in turn, provide a regulated DC output 106 to the load 107. The rectifier 101 may be a bridge rectifier that provides full-wave rectification of the AC input source 104. Additionally, the unregulated DC output 102 may be filtered with a bulk capacitor 103 coupled to the output of the rectifier 101.
A conventional flyback power conversion stage 105 typically includes a transformer that provides galvanic isolation between the primary side and the secondary side, and a primary-side switch for electrically coupling or decoupling the load to the unregulated DC output 102, and a switch controller coupled to the switch for controlling the on-time and off-time of the switch. Energy from the unregulated DC output 102 may be stored in the gap of the transformer when a switch is on and is transferred to the load when the switch is off. The switch controller controls the switch to be turned on or off with on-times or off-times that are adjusted at the operating frequency of the power converter according to the adopted regulation scheme, such as pulse width modulation and/or pulse frequency modulation, in order to regulate the output voltage 106 provided to the load 107. In many cases, switching power converters 100 are required to operate over a “universal input range”, allowing for the worldwide operation of the electronic devices. Variation in the AC input source 104 can lead to changes in the output 106 for a given on-time and off-time of the switch. Accordingly, the on-time and off-time of the switch may be modified by the controller based upon a feedback signal (e.g., reflecting the output voltage) to provide a regulated output 106 to the load 107.
FIG. 2A illustrates waveforms for an example universal AC input 104 operating range (V_IN_AC) that may fluctuate between 90 VAC to 270 VAC. As described above, the bridge rectifier 101 converts the V_IN_AC into an unregulated DC output 102 and the bulk capacitor 103 filters the unregulated DC output. FIG. 2B illustrates waveforms for an unregulated DC operating range (V_IN_DC) corresponding to the V_IN_AC range of FIG. 2A. The resulting DC input voltage of power conversion stage 105 effectively extends from a minimum value (V_IN_MIN), at the zero crossing of the AC input voltage while set at 90 VAC, to a maximum value (V_IN_MAX), at the peak of the AC input waveform while set at 270 VAC.
In order to insure proper operation, it is desirable for a switching power converter 100 to monitor the unregulated DC input 102 of the power stage 105. In conjunction with monitoring the unregulated DC input 102, additional protection and safety features may be included should the DC input voltage deviate from the specified operating range (e.g., exceed V_IN_MAX or drop below V_IN_MIN) and/or when the switching power converter 100 has become uncoupled from the AC input voltage source.
FIG. 3 illustrates an example block diagram of a flyback power supply 300 where the waveforms of the unregulated DC voltage 302 are consistent with those shown in FIG. 2B. As shown, a bridge rectifier 303 is coupled to the AC input (not shown) and input bulk capacitor 304 is coupled across the output of the bridge rectifier 303 to filter the unregulated DC voltage 302. The unregulated DC voltage 302 is input to the power stage 305, which includes power transformer 309, controller 301, and switch SW. Controller 301 is coupled to the switch SW and regulates the DC output 306 by controlling the on-time and off-time (e.g., the duty cycle) of the switch SW at an operating frequency of the switching power converter 300. Controller 301 may also sense DC voltage 302 at voltage sense pin V_IN. One commonly used technique is to sense the magnitude of the DC voltage 302 (or a scaled waveform thereof from a voltage divider) using an analog-to-digital converter (ADC). Monitoring of the magnitude of the DC voltage 302 at the V_IN pin of the controller 301 may enable detection of a brown-out condition and a loss of AC source condition (e.g., due to a decoupling of the AC source from the rectifier 303).
Brown-out conditions represent potentially damaging conditions for power supplies 300, especially when operating in areas where AC sources are unreliable. In brown-out conditions, the AC input voltage drops to a level that is below the specified operating range (e.g., 90-270 VAC) of the power converter 300. For example, in areas where the AC input voltage range is 90 VAC to 130 VAC, brown-out conditions exist when the AC input voltage drops below 90 VAC. As a result, the DC voltage 302 input to the power conversion stage may drop below the specified operating range of the power conversion stage 305. Continued operation in brown-out conditions can lead to a number of undesirable results, including increased thermal conditions of power converter 300 components, power converter component failure and damage, and damage to the load 107 due to loss of regulation of the output 306. Often times, a brown-out condition is brief, lasting only a few AC cycles. However, when brown-out conditions persist over longer periods of time, the power supply may suffer permanent damage.
FIGS. 4A and 4B illustrate example waveforms for AC input voltage (V_AC_1) and the corresponding unregulated DC input voltage (V_DC_1) of a power stage during a brown-out condition. FIG. 4A shows the AC input voltage (V_AC_1) fall below the minimum specified Peak-Peak level (V_P-P_MIN) corresponding to a brown out condition. FIG. 4B illustrates the resulting DC voltage level (V_DC_1) dropping below the minimum DC input voltage level (V_IN_MIN) (i.e., as specified by a controller).
Loss of an AC source condition may occur when the switching power converter has been uncoupled or unplugged from the AC input voltage source. For example, many switching power converters are used to recharge the batteries of electronic devices such as cellular telephones and tablet computers. The switching power converter often remains connected to the AC input voltage source once the portable device has been detached from the power supply. In response, the power converter may enter into a “sleep-mode” in which internal power consumption is significantly reduced while the power converter maintains a regulated voltage output in anticipation of a device being reconnected. Often times, the power converter may be unplugged from the AC input voltage source while it is in a “sleep-mode”. As a result of the reduced power consumption, the unregulated DC input voltage may remain within the operating range of the power stage due to a large bulk capacitance 304 for an extended period of time.
FIGS. 5A and 5B illustrate example waveforms for AC input voltage (V_AC_2) and the corresponding unregulated DC input voltage (V_DC_2) of a power stage during a loss of an AC source condition at time t_1. Similar to the brown out condition, the persistence of unregulated DC input voltage (V_DC_2) below the V_IN_MIN threshold can cause damage to the power supply. Additionally, if the load has been decoupled from the regulated output, maintaining the regulated output after time t_1 by the controller may present a safety hazard for the end user or manufacturing personnel during the test and assembly of the power supply.